Ball valves typically include a body with a through passage for fluid flow and a rotary member located in the body, which has a generally spherical shape. The spherical ball member has a through passage and the valve is opened by rotation of the rotary ball member such that the through passage of the ball is in line with the through passage of the body. Conversely, the valve is closed by rotation of the ball member such that the through passages are no longer in alignment. Typically, in the valve closed position the passageway through the ball member is positioned transverse to the through passage of the body. These valves generally fall into one of two categories, a floating ball valve or a trunnion-type ball valve. A trunnion-type ball valve is often preferred because it has a significantly lower operating torque than the floating ball valve since pressure forces tending to push the ball downstream are supported by a trunnion rather than the valve seat. The trunnion may be supported on bearings that do not have to function as a seal and therefore can be made of a material with better frictional properties.
In the trunnion-type ball valve the seals must be pressure activated and can be made smaller with a reduced ball/seal contact area as the seals do not support the massive pressure forces pushing the ball downstream. The pressure activated seal can be designed so that the activating forces are no greater than required to affect a good seal, which can result in an improved seal life.
In certain applications, the trunnion-type valve has an additional advantage in the fact that when the ball valve is in the closed position the upstream seal is under line pressure while the downstream seal is pressure relieving, eliminating the possibility of a pressure lock in the center cavity.
Typically, seals between the valve body and valve ball are spring actuated whereby the seal is biased into compression against the ball, with the spring producing the initial low pressure sealing force. In certain prior art devices, the spring loaded seal and carriage fits into a counterbore in the valve body before assembling the ball. During ball assembly, it is necessary to force the seal back, compressing the spring far enough for the seal to clear the ball when it is assembled in the body. The spring then forces the seal toward the ball to affect a low pressure seal. In such devices, the spring is compressed at assembly well beyond the working load and requires a much lower spring rate. Further, in such devices, the seal carriage must effect a seal with the body and must be made to close tolerances to function at high pressures.